Oil and gas wells extend from the surface to one or more subterranean formations of rock containing oil and/or gas. The well is typically cased by cementing a steel or other suitable casing in the wellbore. The casing stabilizes the sides of the wellbore, prevents pollution of fresh water reservoirs and/or prevents fluids from zones other than oil and gas producing zones from entering the wellbore.
Cementing operations pump wet cement slurry down a wellbore to fill the space between the casing and the rock walls. The cement protects the casing and prevents water and other fluids from flowing vertically in the space between the casing and rock walls of the wellbore. Typically, cementing operations are designed and supervised by engineers. Laboratory technicians test and select the cement slurry and additives.
Cement compositions are designed for a variety of wellbore conditions, which may vary in depth, temperature and pressure. In designing a cement composition for a wellbore, a number of potential slurries are typically tested in a laboratory for pumpability, safe placement time and compressive strength. The compressive strength is used as a measure of the ability of the cement composition to provide long-term zonal isolation. Field observations have shown that during life of the well many cemented wells failed as seen by buildup of pressure behind a cemented pipe, leakage of fluids from the formation into the wellbore via cracks and from channels generated in areas where the cement debonded from the casing or formation. Such problems may be remedied by squeezing sealants, such as cement slurries or silicate fluids, into the flowpaths and allowing them to set. Cement sheath failure may be due to cyclical pressure and temperature changes experienced by cement sheath from various well operations such as completion, pressure testing, fracturing and remedial operations. Simulation of cement sheath durability in realistic, large-scale wellbore models has been performed using a full-scale wellbore model in which cement is pumped between a full-size liner and a pipe representing the subterranean formation of the wellbore. The full-scale models have been performance tested by cycling pressure and temperature under wellbore conditions.